This invention relates to a method for manufacturing a magnetically attractive or magnetizable toner material utilized for an electrophotographic dry development process, and more particularly, to a method for manufacturing a toner material wherein minute particles of magnetic or magnetizable material are arranged to be partially exposed from outer boundaries of the respective toner particles as mentioned above.
Recently, in an electrophotograhic process involving dry development, several methods of the dry development employing toner material including therein magnetizable materials in a manner as described above have been proposed.
According to one method, for example, disclosed in Japanese Laid Open Patent Application (Tokkaisho) No. 51-26046(1976), as the toner material to be utilized for the development, a one-component toner material constituted by toner particles including therein magnetizable materials in a manner as described above is prepared for the abovementioned purpose. Furthermore, according to another method, for example, in U.S. patent application Ser. No. 925,699, there is disclosed a magnetic brush development, in which, as the toning agent for developing, a mixture of toner particles including therein magnetizable materials in the manner as described above and non-magnetizable toner particles is prepared for the above-described purpose.
More specifically, according to a previous method, the toner material, as a whole, is first triboelectrically charged through relative rubbing movements of the magnetizable materials exposed from the respective toner particles and the resin material constituting respective toner particles, and thereby, the development is to be accomplished through an electrostatic force properly generated between the toner material triboelectrically charged in the manner described above and an electrostatic latent image formed on a photoreceptor.
In the latter method, minute toner particles including therein magnetizable materials in a manner as described above, respectively, are first mixed with non-magnetizable attractive toner particles having a minute diameter, respectively, so that the toner material thus mixed up is employed for the development process under a condition wherein respective non-magnetizable toner particles are electrostatically attracted or adsorbed or the magnetizable materials exposed in the manner as described earlier. The mixture mentioned above shows a specific behavior when employed in the developing process. More specifically, respective non-magnetizable toner particles are to be slipped from the respective specific positions whereat these have been attracted by the respective magnetizable materials, due to occurrence of mechanical force generated to a certain extent in the course of the development process and exerted on these mixed particles, and thereby, the mixture material, as a whole, exhibits a relatively electrically, conductive nature which can now serve to make it possible to accomplish such development method of the charge induction type as described in U.S. Pat. No. 3,909,258. However, in the course of the transferring process, since non-magnetizable toner is to cover the respective magnetizable material exposed therewith, the mixture, as a whole, has relatively insulating nature, whereby a corona transferring process is effected.
According to the respective development methods as described in the foregoing, the most substantial condition to accomplish these respective methods mentioned above is that, the magnetizable materials included in the respective toner particles of the above-described type are arranged to be positively exposed from the respective outer boundaries of toner particles, since the exposed magnetizable materials should closely contact either the resin portion of toner particles including the magnetizable materials or the non-magnetizable toner particles. Conventionally, since the toner particles including therein exposing magnetizable materials in a manner as described above are manufactured through a process including following sequential steps of mixing magnetizable materials with resin material to such a extent as these being apparently homogeneously mixed up, and subsequent crushing and spray-drying a resultant mixture, the exposing ratio of the magnetizable materials from the respective toner particles, as a whole, depends upon the proportional amount of the magnetizable materials included in the mixture prepared in advance. Specifically, a probability of exposure of the magnetizable material from the respective boundaries of toner particles is confirmed by the relative amount of the magnetizable materials to be mixed with the rest, i.e., the resin material, in advance. Therefore, if the magnetizable materials are to be precisely exposed from the respective outer boundaries of the respective toner particles, a considerable amount of magnetizable materials are first arranged to be included inside respective toner particles themselves.
As far as the resin material utilized for toner particles to be included therein magnetizable materials to be exposed is concerned, the selection of the material mentioned above is limited to those having a relatively high softening temperature, so that the toner material in use for the above-mentioned purpose should not be easily aggregated during the storage or the processing. The resin material having a relatively high softening temperature as mentioned above is, however, apt to show a tendency to poor fixing ability as the relative amount of the magnetizable materials is to be increased. Moreover, according to the magnetic brush development method, the developing agent utilized for the development is a mixture of magnetically attractive toner material and non-magnetizable toner material. Due to a small degree of adhesion between the magnetically attractive toner material and non-magnetizable toner material, as well as the relatively easy scattering of the toner material during use despite the small particle diameter thereof and because the magnetically attractive toner material has few magnetizable materials which are precisely exposed from the respective outer boundaries thereof, there has been such a disadvantage that fogging of the copied image tends to take place.